Should I ask the user to enter their age or their birthday?

1. How do you ask for your date of birth in English?
2. What's your birthdate?

How do you ask for your date of birth in English?

Here are some questions which you can ask. “When does your Birthday fall?” or “What date was your Birthday?” or “What day was your Birthday?” or “What month was your Birthday?” or “What year was your Birthday?”

What's your birthdate?

Your birthdate is the same as your date of birth.